1. Field of the Invention
The present invention relates to a dynamically damped pulley and, more particularly, to a damper pulley which is lightweight and can be produced in a simple manner.
2. Background and Description of Related Art
Conventional damped pulleys have been constructed from cast metal, central portion, a cured rubber section on the periphery of the damper pulley and a cast ring surrounding the pulley and the rubber section. Such a pulley has significant weight because the central portion and the belt retaining portion are formed of thick cast metal. Also, the cost of curing the rubber or elastic member is expensive and can also require use of adhesives.
FIG. 7 illustrates another conventional damper pulley. The damper pulley comprises a central portion, a belt retaining portion and a rubber or elastic member therebetween. In order to reduce both the weight and production costs, the central portion and the belt retaining portion are formed from thin metal plate material. The central portion includes an axially extending tubular boss portion 10, a main portion 20, extending radially outwardly therefrom, and a flange 210 that is bent at about 90.degree. from main portion 20 so as to extend axially from the outer periphery of the main portion 20. The boss portion 10, main portion 20 and flange 210 define a circular space S1 having a substantially U-shaped cross-section. A ring-shaped belt retaining portion 30, which has a generally U-shaped cross-section interior open space S2 reverse to the space S1, is also formed from thin metal plate. The belt retaining portion 30 is positioned over the outside of the flange 210, and an annular rubber or elastic member 40 is disposed between the flange 210 and the belt retaining portion 30.
Japanese Utility Model application laid-open No. Hei 272835 discloses a damper pulley which combines the above mentioned construction with a second rubber vibration isolator member and a mass-member. Accordingly, the damper pulley comprises a damper pulley portion, a belt retaining portion, a first elastic member disposed between the damper pulley portion and the belt retaining portion, a mass member, and a second elastic member positioned between the inner wall of the belt retaining portion and the mass member.
However, the damper pulley in FIG. 7 is still not satisfactory. The back interior side of the outer wall 310 of the belt retaining portion 30 needs to be supported by a die when grooves 312 are formed by spin forming the surface of the outer wall 310. However, as the outer diameter of the damper pulley is fully formed, the open space S2 may not be satisfactory for inserting the positioning die in case of forming grooves in the outer wall of the belt retaining portion. When the space S2 becomes larger to permit the easy insertion of a support die, the diameter of the rubber or elastic member 40 becomes small so that the torsional spring force deteriorates, and the resonant frequency of the damper pulley depreciates. To overcome the above problems, the hardness of the rubber elastic member needs to be raised, but without lessening the durability thereof. Besides, the elastic member 40, which is disposed between the flange 210 and the belt retaining portion 30, is forced there between to prevent it from falling out. However, this is done by enlarging the diameter of the flange 210 which can result in the remote possibility that cracks might appear on the surface of the main portion 20.